Over time, the industry has learned a great deal about the properties of silicon crystals used to achieve Moore's Law. For example, about 20 years ago, warp engineering became crucial in transistor development. This technology uses layers of materials to locally distort the silicon crystal lattices in order to improve the mobility of electrical carriers through these channels. This type of crystal engineering has helped reintroduce non-silicon and composite semiconductors to the industry, which were previously considered too difficult to use, expensive, or brittle, despite their superior electrical properties.
Manufacturing processes and materials are fundamental to progress
Gallium nitride (GaN), for example, was investigated as a semiconductor material half a century ago, but devices made with this material required the use of expensive and brittle sapphire substrates. Today, GaN films can be reliably and flawlessly developed on the surface of a silicon wafer. This has paved the way for a growing supply of high-performance devices. Similarly, improvements in manufacturing processes have led to silicon compounds such as silicon carbide (SiC), which is nearly as hard as diamond and practical for high-volume manufacturing.
GaN and SiC semiconductors are characterized by band gaps between the valence and conduction bands that are three times wider than those of conventional silicon devices. This, along with other properties of these semiconductors, makes them ideal for high-power circuits. The materials exhibit critical field values ten times higher than those of typical silicon devices. The result is a higher breakdown voltage, which can be used to reduce the size and cost of transistors used in power supplies and other circuits that require the ability to handle high voltage and current levels.
Due to the higher critical field values and breakdown voltage, the drift region of a vertical transistor or diode can be much narrower when using SiC than with a silicon device. This not only leads to a reduction in on-resistance, resulting in significantly lower power losses when current flows through the device, but also to shorter recovery times since there are fewer minority carriers to clear, for example, in a reverse-biased diode. A device like the WeEn Semiconductors WNSC021200 has a recovery charge of only 10 nC, much lower than that of silicon PiN diodes.
Size and mobility as catalysts for future applications
In turn, the smaller size of the transistor allows for a reduction in parasitic elements such as gate and output capacitance. The sum of these effects results in higher switching frequencies in switched-mode power supplies and similar circuits. GaN devices, for example, can handle switching frequencies well above 1 MHz. At these frequencies, it is also possible to use smaller passive components, which are typically used to smooth switching transients. The result is a smaller and more efficient power supply.
Some manufacturers, such as Power Integrations, have leveraged the properties of GaN to create integrated circuits for power supply switches. One example is the INN3276C-H204, an offline flyback switching IC with an integrated primary-side switch, ideal for constant-power battery charging applications. This device supports up to 35 W of output power; other products in the company's PowiGaN family can support up to 100 W of output power without requiring a heatsink.
Another attribute of GaN is its inherently superior carrier mobility: almost 40% greater than that of silicon. This high mobility results from the way a two-dimensional electron gas forms at the interfaces between the component's materials, an intrinsic characteristic of HEMT (high electron mobility transistors) and also found in other materials, such as gallium arsenide (GaAs). This high mobility characteristic has led to the use of GaN in applications such as base stations and scientific instrumentation. Nexperia devices are specifically designed for this market sector.
Beyond RF applications in cellular communications, there is a growing interrelationship between RF design and power supply in the rapidly expanding field of wireless charging. The AirFuel standard, for example, requires a transmission frequency of 6.78 MHz. Devices such as Infineon Technologies' CoolGaN power transistors are well-suited to the Class D and Class E amplifier topologies being used in these applications.
SiC's ability to handle higher switching frequencies than silicon has led to its use in high-voltage medical equipment such as X-ray machines, among other power handling applications. The near-instantaneous switching capabilities of SiC MOSFETs, such as ROHM's SCT3 series, are complemented in the xR series by the modified TO-247 package's four-pin configuration. This design is used in components like the SCT3060ARC14, a 39 A, 650 V MOSFET, to reduce switching losses by up to 35% compared to conventional three-pin package configurations. A separate source-drain connection from the gate driver helps minimize parasitic inductance, which can be leveraged to increase the circuit's switching speed.
Another key advantage of these wide-bandgap semiconductors lies in their ability to operate at high temperatures, which offers significant size and cost benefits. For example, transistors do not require large heat sinks to dissipate heat, unlike silicon MOSFETs, which need them to avoid losses due to higher conduction resistances and elevated temperatures. SiC surpasses GaN by offering the ability to operate under conditions beyond the capabilities of silicon devices. This has made SiC an increasingly popular choice for circuits in automotive, military, and drilling applications, as it can be mounted near the engine compartment, brakes, or drill heads and retain functionality when temperatures reach 200°C. To withstand these higher temperatures, these devices are often supplied as exposed chips for integration with specialized high-temperature packages. However, many of those in widely used packages, such as DPAK or TO-247, created by Genesic Semiconductor, Littelfuse, and OnSemi, among others, operate at junction temperatures up to 175 °C.
Intelligent control and how to better implement GaN, SiC and relatedtechnologies
The high switching frequencies that wide-bandgap semiconductor materials allow designers to leverage novel control techniques such as array conversion, which uses space-vector modulation calculations for controlling high-performance AC motors in an architecture that can be implemented in a wider variety of power supply systems as an alternative to the higher switching speeds of conventional topologies. Gate driver performance is equally important in all these fast-switching circuit architectures. In addition to creating the CoolGaN range of power devices and the CoolSiC transistors and diodes, Infineon has invested in technologies such as EiceDriver. EiceDriver is a gate driver design that employs high levels of common-mode rejection to enable the use of non-isolated topologies in a wider variety of applications, thereby lowering bill-of-materials costs.
The need for intelligent control is driving the development of highly integrated power devices that combine microcontrollers and power transistors, leveraging advances in sip-based (SiP) manufacturing. STMicroelectronics recently announced the STi2GaN initiative, which integrates GaN power stages and control logic using BCD, along with controllers and isolation circuitry, into a single SiP. The tight coupling of these circuit elements helps push power supply designs into the megahertz range and reduce the overall board footprint, which is becoming increasingly important in electric vehicle designs where many components must be assembled in parallel, as well as in consumer battery chargers.
As widebandgap materials become better understood, their integration with silicon circuits will drive a variety of applications. It's easy to see why many conventional silicon devices are being replaced in numerous areas. An experienced technical distributor like Farnell can guide engineers on where GaN, SiC, and related technologies can be most advantageously used and how device manufacturers are evolving to incorporate them into new applications.
By Cliff Ortmeyer, Global Head of Technical Marketing at Farnell
